Crimean-Congo Hemorrhagic Fever virus (CCHFV), a member of the genus Nairovirus of the family Bunyaviridae, causes severe disease in humans with high rates of mortality (50%). Humans can become infected through bites of the Ixodid tick, by contact with a CCHFV infected patient during the acute phase of infection, or by contact with blood or tissue from viremic livestock. Based on the high mortality rate of CCHFV and the potential for CCHFV dissemination by aerosolization, CCHFV is considered to be of potential bioterrorism importance and is classified as a Category B agent. Because of the requirement to work with CCHFV under BSL4 conditions, virtually nothing is known about its cell biology, the mechanisms by which it enters cells, the antigenic structure of its glycoproteins or the mechanisms by which antibodies can neutralize the virus. The RNA genome of CCHFV consists of three negative sense segments: the small (S), medium (M) and large (L) genomic RNA segments. The CCHFV RNA M segment encodes the virus glycoproteins, G1 and G2. The G1 protein is unusual in that it has an N-terminal mucin-like domain that is cleaved from the protein post-translationally, a second N-terminal domain that is also cleaved from the protein, and we have recently found that a C-terminal cleavage event occurs as well, resulting in release of much of the cytoplasmic domain of the protein. Over the past year, we have initiated a collaborative study of CCHFV with Dr. Connie Schmaljohn (USAMRIID) and Dr. Adolfo Garcia-Sastre (Mr. Sinai). Our long-term plan is to prepare a Program Project grant to support our work. The intent of this R21 application is to seek support for studies in the Doms lab that will provide the preliminary results needed for the P01 application. In this R21 application, we propose the following three Specific Aims: Aim 1. Clone and analyze CCHFV M protein segments from geographically diverse CCHFV isolates, and develop the expression systems and tools needed to study and compare the G1 and G2 proteins. Aim 2. Characterize a novel panel of monoclonal antibodies directed against the CCHFV G1 and G2 proteins, including a number of neutralizing antibodies. Aim 3. Develop virus pseudotype systems or cell-cell fusion assays that can be used to study CCHFV tropism, fusion and entry mechanisms under BSL2 or BSL3 conditions.